A question of balance?

Okay - so I plan built a small flying wing wing and spent an hour today, getting its C of G the right distance from the nose, and also ensured it was balanced perfectly left and right (sure someone will give me the right term for being balanced left and right).

Gave it a couple of test chucks this afternoon - problem is as soon as it left my hand it rolls violently to the left then takes a nose dive. - infact it completely inverted itself on my first throw before hitting the deck - hard!
No stick input or max trim will stop it rolling to the left, so does this mean I need to add lead to the light side, even though it will throw out the overall left/right balance?

If so - how much lead should I be adding or is a just case of trial and error?

I may not be the best builder in the world, but Im happy with the build quality and know the aerofoil is even - cant really understand why it should react so violently in the air! - its almost like when you mis-throw a frizbee and it hits the deck three feet infront of you!

Instant thought from me is:

Is it Lateraly balanced i.e balance the nose and the center of the rear and see if the left wing drops

M

Just re read your post and you have sorry,

Erm that case have you got any reflex in on the elevators

M

Yeah - clamped a steel ruler on my bench this afternoon and sat it on that to balance it up! added about 15g to the right wing to get it spot on!

Just had a thought though - when adding lead to the nose, I glued it in as far forward as I could get it, but glued it to the top of the areofoil not the bottom like this:-



Would that make any difference?

Mark Barnes wrote:you got any reflex in on the elevators

M

Just for a laugh - pretend I have no idea what you are talking about and explain what you mean in a little more detail! - Just for a laugh of course!

most flying wings require abit of up elev on to fly and make the section work this is called Reflex. experiment with a few mm addin more each "test" flight.

I recal my Weasal required an awfull lot

M

Gotcha chief! - didnt realise it had a specific terminology - I would have just called it "up trim"!
Have got some reflex in, but maybe not enough - will try that next.

Could do with finding somewhere with some long grass now to save the hard landings during test chucks!

Dont know if you know or not but RDMFC have a loverly field, maybe you could visit somtime

HEHE

M

If it has flat bottom wing section put the steel rule from front to back, LE to TE & set the elevons level with the rule, should show quite a lot of up trim if that's what you want to call it, one of the things with flying wings is not to launch it too hard just a gentle push is all it takes. If it's symmetrical section set reflex to approximately 5mm up.

It will roll violently if you launch too hard.

Don't worry too much if the elevons aren't even after it's trimmed as long as it fly straight & level.

It doesn't matter where the lead is as long as it's balanced.

Do you have the same incidence on both wings?

Warp on one side (twist in wing ) ???

Are you sure the CG is in the right place? Quite often a rearwards CG, mainfests itself in an extremely violent response to control inputs, so even just putting a smidge of aileron in, results in a stall and a flick. Laucnhing too hard would magnify that behaviour too....

Andy

Allan - same incidence on both sides
Gary - wing is perfectly straight not warped
Andy - C of G is to the plan (12.5 mm back), but could bring it forward further to see if that helps
Rich - was giving it a bit of welly when launching so might have overdone the wrist action a bit (oo err!)

Will check the exact amount of reflex later when I get home!

Mark, you might have to give me the sat nav coordinates of this mystical so called........"field"

I'm going to bet a bacon butty that if you bring the CG forward, it will calm it down on the launch......

Ben Just make sure that by moving the CG forward it has no effect then you & the rest of us get the bacon butties

Notice that the bet agreed was for "A" bacon button. Singular. Therfore it will be one bacon butty for you all to share.......

Andy

Oi Andy, Ive just found some bright yellow cord on the back of my van!!!!

M

I thought that was green

Andys bondage ropes!

Andy Sayle wrote:Notice that the bet agreed was for "A" bacon button. Singular. Therfore it will be one bacon butty for you all to share.......

Andy

Dont worry - I will save you the crusts Rich!

Mark Barnes wrote:Oi Andy, Ive just found some bright yellow cord on the back of my van!!!!

M

Why that's a might coincidence isn't it? I had some yellowy-green cord yesterday.....

Andy

I bet it needs a tad of reflex
Tailless wings need it to compensate for no horizontal stabilizer ( tailplane ). The centre of pressure or lift, being slightly behind the centre of gravity, tries to "roll" or more correctly "pitch" the wing LE down slightly... the tailplane normally counteracts this moment, but of course, you dont have a tailplane, so the reflex acts like a bit of permanent "up" elevator to pitch it back down level again.

Hey, just another thought - the side fins don’t come below the bottom of the wing as on other wings, they just sit on the top like shark fins. Would it be worth altering the side fins so they protrude below the bottom edge?

They don’t need to according to the plan, but what do you lot think?

I doubt it will make any difference to your "problem"
Increasing the area of fin ( either under or over the wing top surface ) will probably increase yaw stability - where the model appears to want to wobble from side to side, or simply doesn't handle well in the turns.
It wont have any effect on pitch stability - a little fin protruding from under the wing may help to protect the servos and horns etc from ground damage ( assuming they are underneath )
Having winglets on the ends of the wing can also improve airflow problems over the wing, as it were "trapping" the air and stopping it "spilling off" the tips which creates turbulence etc. A bit like the job of wing fences which are seen on some models.
However, if this was required on that particular wing section then Im sure the designer would have incorporated them.

Wingtip (i.e. fins on the wingtips) design is a whole different kettle of fish, and as Tim says, it is going to make naff all difference to the CG position. You can do some funky things depending on the planform of the wing, and the aerofoils used. The most beneficial one being to alter the formation of wingtip vortices, which allow the stall behaviour to be changed for the better (or worse if you get it wrong!) Getting the design right to do this is horribly complex, and in terms of model aircraft it is best left to "that looks pretty, we'll go with that shape" and accept that it may have a miniscule effect on the overall handling. So miniscule, even Rich couldn't tall the difference

Andy